1. Field of the invention
The present invention is directed to transmission and reception antennas for use in a nuclear magnetic resonance (NMR) apparatus and in a magnetic resonance imaging (MRI) apparatus, for calculating spectra or generating images of an examination subject or object.
2. Description of the Prior Art
Nuclear magnetic resonance (NMR) is the effect of a resonant rotating or alternating magnetic field, imposed at right angles to a static field, to perturb the orientation of nuclear magnetic moments.
Magnetic resonance imaging (MRI) is the development of nuclear magnetic resonance (NMR) techniques for obtaining a diagnostic scan of a subject such as a human or animal body.
The patient or examination subject is placed in a strong magnetic field. This magnetic field induces a net magnetization of the nuclei in the subject. A short radio frequency pulse (RF pulse) is applied at the Larmor frequency of the precessing nuclei which, in turn, emit an RF signal of the same frequency. This is detected and gives a "fingerprint" of the environment of the nucleus being studied. This information is one-dimensional but is converted into a two-dimensional anatomical image by adding a gradient to the applied magnetic field which results in a frequency modulation of the emitted RF signal. A series of such measurements is analyzed by computer to generate the image.
In an NMR apparatus, as well as in an MRI apparatus, a strong magnetic field aligns the nuclei of the subject or object to be studied along the field direction. A radio frequency field delivered through a transmitter antenna, also known as the transmitter coil, brings the nuclei to a higher energy state.
A receiver antenna, also referred to as a receiver coil, intercepts the signal emitted by the object (patient) as its nuclei precess or relax from the higher energy state.
The conditions of efficient energy absorption require that the RF fields of the transmitter and receiver antennas be orthogonal to the static magnetic field. The simultaneous condition of negligibly low interaction between the receiver and transmitter antennas for minimal mutual distortion of their RF fields and for the prevention of high transmitter power to be applied to the receiver, in part also means that the RF fields of the transmitter and receiver antennas must be orthogonal even though transmission and reception do not coincide in time. Acceptable switching time between the two is of the order of about ten microseconds.
As represented by FIG. 1, which shows the prior art, a radio frequency (RF) transmitter 4 transmits a signal to a transmitter antenna 2. The signal which is absorbed and subsequently reemitted by the examination body or subject (patient) 5 is then picked up by a receiver antenna 1 and fed to a radio frequency (RF) receiver 3. The examination body or subject 5 which is being scanned is within the RF fields of both the transmitter and receiver antennas 2,1. The transmitter antenna 2 and receiver antenna 1 are optimized for their respective tasks and are tuned by variable capacitors 6 and 7. Normally, the single, fixed in place transmitter antenna 2 is big enough to encompass the largest part of a body or subject 5 to be scanned. Significantly smaller reception antennas 1, specialized for different body parts, are separately connected and used. Magnet poles 8 and 9 are shown located above and below the patient (body or subject 5), respectively.
Because of the orthogonal arrangement and spatial separations of the transmitter antenna 2 and receiver antenna 1, the resulting low electromagnetic coupling between the transmitter antenna 2 and receiver antenna 1 assures low field distortion and allows utilization of speedy and efficient electronic tuning for the receiver antenna 1. The required transmitting power is high and manual tuning of the transmitter antenna 2 by a variable capacitor 6 is employed, for no known electronically controlled semiconductor device may withstand the incident power.
Another limitation of the prior art apparatus is that the magnet gap between magnet poles 8 and 9 should be large enough to receive the two separate antennas 1, 2 therebetween, which is not always possible or cost efficient.
These prior art types of devices are described, for example, in U.S. Pat. Nos. 4,926,126; 4,975,644; 5,144,244; and 5,256,972, the entire contents of which are incorporated herein by reference.
FIG. 2 represents another prior art apparatus using a single antenna 10 which is alternatively switched by switch 14 between the RF signal transmitter 12 and RF signal receiver 13. The antenna 10 is tuned by a variable capacitor 11.
In the case of FIG. 2, the transmitter power requirement is significantly lower as compared to the embodiment of FIG. 1. The antenna 10 is not limited to a single orientation but may be multi-orientable. The antenna 10 also does not put a large constraint on the size of the magnet gap. These prior art types of devices are described, for example, in U.S. Pat. Nos. 4,901,022 and 5,166,617, the entire contents of which are incorporated herein by reference. However, even though the transmitter power is lower in the embodiment of FIG. 2, the transmitter power requirement is still so high as to prevent simple implementation of electronic antenna tuning.